Hydrocarbonaceous gases such as, for example, natural gas or liquid gas, in addition to the usually naturally occurring sulfur compounds, also comprise sulfur compounds which are added to these gases for safety reasons.
On an industrial scale, natural gas is predominantly desulfurized by catalytic hydrogenation with addition of hydrogen. However, this desulfurization method cannot be used appropriately for small and very small applications, especially fuel cells in the domestic sector, and so in this case use is chiefly made of an adsorptive method for purifying the gas stream.
The hydrogen necessary for operation in fuel cells is usually obtained by reforming natural gas. Natural gas possesses, especially in highly industrialized countries, the advantage of wide-scale availability, since a fine-meshed supply grid exists. In addition, natural gas has a hydrogen/carbon ratio which is expediently high for hydrogen generation.
The expression “natural gas” describes a multiplicity of possible gas compositions which can vary greatly depending on the locality. Natural gas can comprise virtually exclusively methane (CH4), but can also comprise considerable amounts of higher hydrocarbons. The expression “higher hydrocarbons” is taken to mean here all hydrocarbons from ethane (C2H6), regardless of whether these are linearly saturated or unsaturated, cyclic or aromatic hydrocarbons. Typically, the fractions of higher hydrocarbons in the natural gas decrease with higher molecular weight and higher vapor pressure. For instance, ethane and propane are usually found in the low percentage range, whereas hydrocarbons having more than ten carbon atoms are usually present in natural gas only at a few ppm. Among the higher hydrocarbons are also cyclic compounds such as, for example, the carcinogenic benzene, toluene and xylenes. Each of these compounds can occur in concentrations of >100 ppm.
In addition to the higher hydrocarbons, other minor gas components and impurities which may comprise heteroatoms occur in natural gas.
In this context, in particular, mention may be made of sulfur compounds of natural origin which can occur at low concentrations. Examples thereof are hydrogen sulfide (H2S), carbonyl sulfide (COS), carbon disulfide (CS2) and light organosulfur compounds such as, for example, MeSH. Depending on the origin of the gas, the COS which is difficult to remove, especially, can occur at an elevated concentration which makes a complex purification stage necessary.
In addition to the sulfur compounds naturally present in natural gas such as, especially, H2S and COS, other sulfur compounds are added to the natural gas for safety reasons as what are termed odorants. Methane and natural gas are odorless gases per se which are nontoxic but, in combination with air, can lead to explosive mixtures. In order to be able to detect an escape of natural gas immediately, natural gas is admixed with intensely odorous substances at a low concentration which, as what are termed odorants, give the characteristic odor of natural gas. The odorization of natural gas is prescribed by law in most countries—together with the odorants which are to be used. In some countries such as, for example, the United States of America, mercaptans (R—S—H, R=alkyl moiety) such as tert-butylmercaptan or ethylmercaptan, are used as odorants, whereas in the member states of the European Union, usually cyclic sulfur compounds such as tetrahydrothiophene (THT) are used. Owing to a possibly proceeding chemical reaction, from these mercaptans (R—S—H), sulfides (R—S—R) and/or disulfides (R—S—S—R) can be formed which must likewise be removed. Together with the naturally occurring sulfur compounds, therefore this gives a multiplicity of different sulfur compounds in the natural gas. The differing regulations for the composition of natural gas usually permit up to 100 ppm of sulfur in the natural gas. The situation with liquid gas (LPG) as feedstock is similar. Liquid gas which comprises, as main constituents, propane and butane, must, just as natural gas, be admixed with sulfur-comprising molecules as odor markers.
The sulfur components in the natural gas or LPG can lead to strong and irreversible poisoning of the catalysts in the fuel cell or in the reformer. For this reason the gases which are fed to the fuel cell must be purified from all sulfur-comprising components. Fuel cells, for this reason, always comprise a desulfurization unit for the natural gas or LPG used. Should the fuel cell be operated with liquid hydrocarbons such as, for example, heating oil, desulfurization is likewise necessary.
Preference is given to a process procedure in which the hydrocarbonaceous gas is passed in straight throughflow at room temperature through an adsorber which completely removes virtually all sulfur components. The adsorber should preferably operate at the operating temperature and operating pressure of the fuel cell. For safety reasons, the adsorber container is generally located in the housing of the fuel cell. Temperatures of up to 70° C. can prevail there. In addition, the pressure in the gas pipe grid at the end user is generally up to several hundred mbar above ambient pressure.
Since the adsorber is intended to be suitable for the operation of natural gases of differing composition, it is in addition of importance that only the sulfur-comprising components are adsorbed from the natural gas and the co-adsorption of higher hydrocarbons is suppressed to a negligible extent.
The co-adsorption of higher hydrocarbons, in particular benzene, from natural gas, can in addition have the consequence that legal limiting values for benzene contents in the adsorber are exceeded and the adsorber unit must then be labeled. Such benzene-saturated adsorbers in addition, e.g. during change of the adsorber medium or during transport of the adsorber to recycling, give rise to considerable increased complexity and costs.
In the current prior art, desulfurization of natural gas succeeds only via a two-stage arrangement of different adsorbents which are used especially for removing the natural sulfur components and the odorants.
US-A 2002/0159939 discloses a two-stage catalyst bed comprising an X-zeolite for removing odorants and, subsequently thereto, a nickel-based catalyst for removing sulfur-comprising components from natural gas for operation in fuel cells. A disadvantage of this process is that COS cannot be eliminated directly, but only after preceding hydrolysis to H2S. In addition, benzene and higher hydrocarbons are taken up by the zeolite. In addition, nickel is known as carcinogenic.
U.S. Pat. No. 5,763,350, for removing naturally occurring sulfur compounds, proposes inorganic supports, preferably aluminum oxide, impregnated with a mixture of oxides of elements of groups IB, IIB, VIB and VIIIB of the Periodic Table of the Elements, preferably a mixture of Cu, Fe, Mo and Zn oxides. Here also, the sulfur compounds are first hydrolyzed to H2S.
According to DE-A 3 525 871, naturally occurring organosulfur compounds, such as COS and CS2, which are present in gas mixtures are quantitatively eliminated using sulfur oxides or nitrogen oxides in the presence of catalysts, wherein the catalysts used are compounds of Sc, Y, of the lanthanides, actinides or mixtures thereof on, e.g., aluminum oxide. The catalysts, during their production, are dried and sintered at 100 to 1000° C.
According to U.S. Pat. No. 6,024,933, the sulfur components are directly oxidized to elemental sulfur or to sulfates on a supported copper catalyst which, on aluminum oxide as support, has at least one other catalytically active element selected from the group Fe, Mo, Ti, Ni, Co, Sn, Ge, Ga, Ru, Sb, Nb, Mn, V, Mg, Ca and Cr.
WO 2007/021084 describes a copper-zinc-aluminum composite as desulfurizing agent, which composite is calcined at 200 to 500° C.
EP-A 1 121 977 discloses removal by adsorption of sulfur-comprising odorants such as sulfides, mercaptans and thiophenes from natural gas using silver-doped zeolites at room temperature.
A further important disadvantage—in addition to the high silver content—of the zeolite-based systems is the fact that zeolites readily adsorb in their pore system all higher hydrocarbons occurring in the gas stream. In particular cyclic hydrocarbons such as, e.g., benzene, are completely adsorbed and can be accumulated in the zeolite up to the range of some % by weight.
The processes of the prior art do not solve the problem of unwanted co-adsorption in the pore system of the catalyst of, in particular, cyclic hydrocarbons occurring in the gas stream such as, e.g., benzene. A further disadvantage is that the adsorption of higher hydrocarbons, in some circumstances, leads to pyrophoric adsorbents, so that, in the presence of an ignition source, they can catch fire during removal of the spent catalyst. A further disadvantage is that COS can generally only be removed with the aid of an upstream hydrolysis stage.